This invention relates to pumps, and in particular, though not exclusively, to a pump for use in the oil/gas and/or chemical industries. A pump of the present invention is particularly suitable for use in a method of “artificial lift” in an oil/gas well.
In many oil wells the oil does not have enough pressure to flow all the way up the tubing to the surface. The produced water and oil has to be lifted up the tubing to the surface by one of several methods, normally called artificial lift. Even with a flowing oil well, as more fluids are removed from the subsurface reservoir, the pressure on the remaining oil decreases until it no longer flows up the tubing to the surface.
A common artificial lift apparatus is the sucker-rod pump system. The sucker-rod pump or rod-pumping system uses a downhole rod pump, a surface pumping unit, and a sucker-rod string that runs down the well to connect them. The sucker-rod pump has a standing valve and travelling valve. The travelling valve reciprocates up and down while the standing valve remains stationary.
The sucker-rod pump system suffers from a number of problems. Fluid pound is a problem caused when the produced liquid is pumped faster than it is flowing into the well. Gas enters the pump and the pump can be damaged. Gas lock is an extreme case of fluid pound. Gas accumulates in the pump and prevents the pump from working.
An artificial lift method used on wells that produce large volumes of liquid is gas lift. In a gas lift well, a compressed inert gas called lift gas (usually natural gas that was produced from the well) is injected into the annulus in the well between the casing and tubing. Gas lift valves—pressure valves that open and close—are spaced along the tubing string. They allow the gas to flow into the tubing, where it dissolves in the liquid and also forms bubbles. This lightens the liquid and, along with the expanding bubbles, forces the produced liquid up the tubing string to the surface where the gas can be recycled. The advantages of gas lift is that there is very little surface equipment and few moving parts. Gas lift is a very inexpensive technique when many wells are serviced by only one central compressor facility. However, it is effective only in relatively shallow wells. Offshore oil wells and crooked or deviated wells that need artificial lift are usually completed with gas lift. Gas lift is either continuous or intermittent (periodically on and off) for wells with low production.
Artificial lift may also be provided by means of a submersible electrical pump. A submersible electrical pump normally uses an electric motor that drives a centrifugal pump with a series of rotating blades on a shaft located on the bottom of the tubing. An armoured electrical cable runs up the well, strapped to the tubing string. Electricity is supplied by a transformer on the surface. The electric motor has a variable speed that can be adjusted for lifting different volumes of liquids. Submersible electrical pumps are used for lifting large volumes of liquid and for crooked and deviated wells. A gas separator is often used on the bottom of the pump to prevent gas from forming in the pump and decreasing the pump's efficiency. Prior art electrical pumps are therefore coupled to a turbine or the like and provide axial flow of fluid.
A hydraulic pump may also be used to provide artificial lift. A known hydraulic pump is identical to a sucker-rod pump except it is driven by hydraulic pressure from a fluid pumped down the well. It uses two reciprocating pumps. One pump on the surface injects a high pressure power oil (usually crude oil from a storage tank) down a tubing string in the well. A reciprocating hydraulic motor on the bottom of the tubing is driven by the power oil. It is coupled to a pump, similar to a sucker-rod pump, and located below the fluid level in the well. The motor drives the pump, which lifts both the spent power oil and the produced fluid from the well up another tubing string. The power fluid causes the upstroke and the release of pressure causes the downstroke. It is called a parallel-free pump. In another variation, (casing-free pump), the power fluid is pumped down a tubing string and the produced liquid is pumped up the casing-tubing annulus. The stroke in a hydraulic pump is very similar to a sucker-rod pump stroke except it is shorter. Hydraulic pumps can be either fixed (screwed onto the tubing string) or free (pumped up and down the well). They can also be either open (with downhole mixing of power and produced fluids) or closed (with no mixing). Most are free and open.
Known pumps used in artificial lift methods suffer from a number of problems/disadvantages—e.g. low efficiency (hydraulic efficiency).
It is an object of at least one aspect of the present invention to obviate or mitigate one or more of the aforementioned problems/disadvantages in the prior art.
It is a further object of at least one embodiment of the present invention to provide a pump which provides a positive displacement of a predetermined volume of well production fluid for each operative cycle of the pump—in contra-distinction to pumps of the prior art which provide axial flow of well production fluid.